Both soil improvement and soil stabilisation consist of adding binders in-situ to improve the soil performance as an alternative to ‘dig and dump’. They have a long track record and are both cost effective and sustainable. The benefits include reduced aggregate importing and off-site disposal and the attendant reduced vehicle movement. Soil improvement is also often used to gain rapid access to wet sites in a single operation.
Soil improvement and soil stabilisation can be used for fills, foundations, subgrade improvement, slope repairs, working platforms and hardstandings.
Soil improvement is generally achieved by adding binders. It is sometimes termed soil modification in the literature.
The benefits Soil improvement is used to address handling or compacting problems resulting from high plasticity or high water content. Such soils can be improved by mixing with lime. Adding lime to clay provides three important benefits:
• Breaking down clay inter-particle cohesion,
• Reduced plasticity,
• Moisture content reduction.
This improves the soil’s ability to be:
• handled by conventional earthmoving plant;
• satisfactorily compacted in layers, especially when the moisture content is wet of optimum;
• trafficked and provide a working platform for subsequent layers; • prepared for further treatment.
Soil improvement should be carried out in accordance with the following: • BS EN 16907 Earthworks Part 4: Soil treatment with lime and / or hydraulic binders • Series 600 Specification for Highway Works, Earthworks • Specific project specification(s)
Lime is the only binder that breaks down clay inter-particle cohesion. This makes soil improvement a necessary first stage in cohesive soil stabilisation where more than one binder is to be used. A mellowing period may be required to allow the lime to break down the clay to achieve both adequate pulverisation and to allow full activation of secondary binders if used. Historically, a mellowing period of between 24 to 72 hours was specified. However, with the efficiency in mixing that modern stabilisation plant provides this can be much shorter depending on the soil properties and the secondary binders used.
Whilst soil improvement is beneficial for conditioning soils there may be limited strength increase. To improve strength it is necessary to go to a second stage termed soil stabilisation.
Soil stabilisation should be carried out in accordance with the following:
• BS EN 16907 Earthworks Part 4: Soil treatment with lime and / or hydraulic binders
• Series 600 Specification for Highway Works, Earthworks
• BS EN 14227 for pavement foundations
• Series 800 Specification for Highway Works, unbound, cement and other hydraulically bound mixtures
• Specific project specification Informative information regarding soil treatment equipment types, sequence and processes are provided in BS 16907 Earthworks Par4: Annexes H and I.
Soil stabilisation results in engineered materials that provide: • improved static load resistance • improved resilient behaviour (stiffness) under dynamic loads • reduced permeability • improved freeze-thaw resistance
The keys to success are using the correct binder and ensuring compaction using good earthworks practice. Reduced costs In-situ treatment of soil can be more cost-effective than traditional ‘dig and dump’ methods which incur transport plus importation and disposal costs.
Soil stabilisation can reduce construction programme times by minimising site preparation and designing out imported materials and unacceptable material disposal. Soil improvement allows wet ground to be dried and strengthened within a very short timescale. This permits working in wet conditions and allows work to restart promptly following rainfall. For the construction programme benefits to be fully realised, longer lead-in time is needed during the design stage for ground investigation and mix design.
Soil improvement and stabilisation offer significant environmental benefits over traditional ‘dig and dump’. Improvement and stabilisation turn unacceptable soils into engineered material. They avoid offsite disposal and importing acceptable fill, which often comprise virgin aggregates. There is no removal of waste materials and corresponding importation of aggregates. This reduces lorry movements on the local road network and therefore greatly reduces traffic congestion and pollution.
The delivery of the above benefits relies upon strict adherence to the relevant technical specifications and standards, plus the full implementation of good practice by a specialist contractor.